1. Field of the Invention
The present invention relates to a fan assembly for cooling a radiator, a radiator-condenser assembly or a radiator-condenser-intercooler assembly of a vehicle, comprising:
a shroud having a through-opening, around which said shroud defines an air conveying ring, and a support which is located at the centre of said opening,
a motor-fan assembly which is mounted in said support and includes an axial fan rotatable with respect to said support about an axis of rotation, and
a noise-reducing annular structure, fixed to the air conveying ring and surrounding said fan, said annular structure having
a plurality of projections which are arranged in succession in a circumferential direction and extend radially towards the axis of rotation of the fan, each projection having a face which is bounded by side edges in the circumferential direction and is directed towards said axis of rotation and is substantially parallel thereto, and
a plurality of portions radially recessed towards the air conveying ring and arranged alternating with said projections.
2. Description of the Related Art
A fan assembly of this type is described in EP 1 914 402. Fan assemblies with different forms of the annular structure for reducing the tonal noise are described by the documents U.S. Pat. No. 5,489,186, U.S. Pat. No. 6,863,496, U.S. Pat. No. 7,481,615 and U.S. Pat. No. 6,874,990.
As is known, in order to attenuate air recirculation effects in the fan assembly it is required to reduce as far as possible the distance between the tip of the fan blades and the air conveying ring or, in the case of a fan provided with an outer ring which interconnects the ends of the blades, the distance between this outer ring and the air conveying ring.
The fan is the main source of tonal noise; since it consists of a fairly large number of rotating blades it produces, by its nature, a “non-stationary lift” which is the main source of noise.
The acoustic combination of fan and conveyor, in terms of combined air volume, has an effect on the spatial distribution and the directivity of the acoustic pressure field produced by the “non-stationary lift” and therefore influences the tonal component of the resultant noise.
The presence of an air layer between fan and conveyor contributes functionally to rotation of the fan without interfering with the structure of the conveyor which supports the fan and ensures a suitable fluid dynamic interaction for obtaining the desired pressure difference between upstream side and downstream side of the fan.
From a fluid dynamic point of view, reducing the combined air volume results in an increase in the efficiency, defined as the ratio between output fluid dynamic power (Q·ΔP) and input electric power (V·I); reducing the abovementioned volume in turn results in an increase in the pressure difference imparted by the system to the recirculation flows which are generated at the tips of the blades, therefore opposing the associated losses.
The existence of an air volume between the conveyor and the fan causes a general increase in the wide-band noise, but in particular intensifies the radial modes which maintain the fluctuations in lift produced by the rotation of the blades; these fluctuations in lift result in generation of localized sound-pressure spectral lobes. Moreover, the presence of a wall surrounding the fan produces sound diffusion and reflection effects which contribute to the fluctuations of the existing acoustic field.
In order to solve these problems, in the past different noise-reduction structures have been proposed: for example those described in the documents EP 1,914,402, U.S. Pat. No. 5,489,186, U.S. Pat. No. 6,863,496, U.S. Pat. No. 7,481,615 and U.S. Pat. No. 6,874,990 mentioned above.
The presence of these noise-reduction structures results in irregularity of the surface of the conveying ring which may have one or both the effects as mentioned:
1) favour diffraction and diffusion of the pressure waves in a plane perpendicular to the axis of the fan, resulting in random spatial distribution of the acoustic pressure waves, with a consequent reduction in the tonal component of the noise generated;
2) induce the formation of vortices inside the spaces which have the effect of reducing the fluid-dynamic cross-section, thus preserving the fluid-dynamic performance (lift/head).